As within any hydrocarbon product whose starting point is crude oil, the degree of purity to which paraffins may be refined covers a wide range from relatively crude to relatively pure. Although each grade of paraffins has commercial use, there are special applications which require a paraffin product of exceptional purity. Certain of these special applications additionally require a paraffin product whose composition is substantially limited to linear paraffins, which may alternatively be referred to as normal, unbranched, or straight-chain paraffins. For example, the manufacture of detergents, in which linear paraffins may serve as the alkyl constituent of sulfonated alkylaryl-and alkyl-sulfonate synthetic detergents. Linear paraffins are preferred in such manufacture because they result in a product having superior detergent properties, which moreover has superior biogradability compared to synthetic detergents manufactured from branched paraffins. Other uses for substantially pure linear paraffins include as ingredients for the manufacture of flameproofing agents; as reaction diluents; as solvents; as intermediates in aromatization reactions; as plasticizers; and for use in protein/vitamin concentrates.
Substantially pure linear paraffins, however, are extremely difficult to obtain Linear paraffins intended for industrial and commercial usage are not produced by synthesis, but are instead isolated from naturally-occurring hydrocarbon sources, and most typically from the kerosene boiling range fraction of natural hydrocarbon feedstocks (as used herein, the term "kerosene range" refers to a boiling point range of between about 182.degree.-277.degree. C.) These feedstocks are made up of a wide variety of hydrocarbon constituents and include, in addition to paraffins, contaminants such as aromatic compounds, and heteroatom compounds such as sulfur- containing compounds, nitrogen-containing compounds, and oxygen-containing compounds (i.e., phenolics). The commercial processes used for separating out the linear paraffin component of such feedstocks are generally not sufficiently precise to yield a substantially pure linear paraffin product. Instead, the separated kerosene range linear paraffin product may contain the contaminants described above in amounts sufficient to preclude use of the product for the special applications referred to earlier.
The principal prior art methods for upgrading kerosene range linear paraffins to substantially pure linear paraffins are mild hydrofining followed by acid treating, and severe hydrofining. While acid treating does remove aromatics from kerosene range linear paraffins, this is not an entirely satisfactory procedure. Acid treating addresses only the aromatics component of a contaminated paraffin stream, without improving product purity with respect to heteroatom compounds. In addition, acid treating raises significant concerns relating to health, safety, industrial hygiene, and environmental quality. Moreover, acid treating can actually increase the levels of sulfur in the final product.
As a general matter, processes are known whereby specific hydrocarbon fractions may be purified and/or isolated from a relatively crude source using solid adsorbents. In these prior art processes a bed of a solid adsorbent material is contacted with a hydrocarbon stream in either liquid or a vapor phase under conditions favorable to adsorption. During this contacting stage a minor portion of the hydrocarbon stream is adsorbed into pores in the solid adsorbent, while the major portion, which may be termed the effluent or raffinate, passes through. Depending on the process and the product involved, the adsorbent may be used either to adsorb the desired product, which is then desorbed and recovered, or to adsorb the undesired contaminants, resulting in an effluent which is the purified product.
In either event, during the contacting stage the solid adsorbent gradually becomes saturated with adsorbed material, which consequently must be periodically desorbed. If the adsorbent contains the undesired contaminants, desorption is necessary in order to free the adsorbent for further removal of contaminants. If the adsorbent contains the desired product, desorption both frees the adsorbent for further separation of the desired product from the hydrocarbon stream, and liberates the desired product from the adsorbent for recovery and, if desired, for further processing. Desorption is generally accomplished by first isolating the bed of adsorbent material from the hydrocarbon stream, and then contacting the adsorbent bed with a stream of a substance which has the effect of displacing the adsorbed material from the solid adsorbent. This substance is referred to as desorbent. Once desorption is completed, the bed of solid adsorbent can again be brought into contact with the hydrocarbon stream.
The efficiency of the adsorption/desorption process is determined by several critical factors, including the precise adsorbent selected; temperature; pressure; flow rate of the hydrocarbon stream; concentrations of feed stream components; and, the desorbent. The prior art in this area demonstrates the complexity, and the high degree of specificity, involved in matching a given feedstock, from which a given product is desired, with a suitable adsorbent/desorbent combination, under appropriate conditions to arrive at a commercially acceptable process.
FLECK et al., U.S. Pat. No. 2,881,862, discloses separating aromatic compounds and sulfur compounds from complex hydrocarbon streams through adsorption onto a "zeolitic metallo alumino silicate," which may be desorbed with linear pentane (see column 5, lines 49-54; column 6, lines 8-12).
KIMBERLIN et al., U.S. Pat. No. 2,950,336, discloses the separation of aromatic compounds and olefins from hydrocarbon mixtures that may also include paraffins, using a zeolitic molecular sieve which may be desorbed by gas purge, evacuation, displacement with an aromatic hydrocarbon, or steaming followed by dehydration (see column 4, lines 38-48).
TUTTLE et al., U.S. Pat. No. 2,978,407, discloses the separation of aromatic hydrocarbons from mixtures which include linear paraffins, isoparaffins, cyclic hydrocarbons, and aromatics, using molecular sieves having pore diameters of 13 Angstroms, which may be desorbed by gas purge and/or evacuation (see column 2, lines 65-70).
EPPERLY et al., U.S. Pat. No. 3,063,934, discloses removing aromatic compounds, olefins, and sulfur from the feed to a naphtha isomerization reactor using a molecular sieve, such as a Linde 10.times. or a Linde 13.times. molecular sieve, which may then be desorbed using the effluent from the isomerization reactor (see column 2, lines 36-41).
EPPERLY et al., U.S. Pat. Nos. 3,228,995 and 3,278,422 both generally disclose the separation of aromatics and/or nonhydrocarbons from saturated hydrocarbons and/or olefins using a zeolite adsorbent. The zeolite is desorbed with a polar or polarizeable substance, which is preferably ammonia, although sulfur dioxide, carbon dioxide, alcohols, glycols, halogenated compounds, and nitrated compounds may be used.
KONDO et al., U.S. Pat. No. 4,313,014, discloses the adsorptive separation of cyclohexene from a cyclohexene/cyclohexane mixture using a type X and/or type Y aluminosilicate zeolite, which may be desorbed with a trimethylbenzene (see column 2, lines 3-11).
OWAYSI et al., U.S. Pat. No. 4,567,315, discloses a process for removing aromatic hydrocarbons from a liquid paraffin. The aromatics are first adsorbed by a type X zeolite molecular sieve material, and are then desorbed using a polar or polarizeable substance such as an alcohol or glycol (see column 3, lines 65-68 and column 7, lines 15-20). In a third step the desorbed aromatic hydrocarbons are washed from the zeolite bed using a solvent such as n-hexane, n-heptane, or iso-octane (see column 7, lines 26-30). MIWA et al., U.S. Pat. No. 4,571,441, discloses separating a substituted benzene from a substituted benzene isomer mixture using a faujasite-type zeolitic adsorbent such as type X zeolite or type Y zeolite. Depending on the nature of the substituted benzene whose recovery is desired, the desorbent used may be toluene, xylene, dichlorotoluene, chloroxylene, or trimethylbenzene; an oxygen-containing substance such as an alcohol or a ketone; or, diethylbenzene (see column 3, lines 35-59).
Russian Patent 1,298,202 discloses a method for removing aromatics from a paraffin feedstock using a solid adsorbent such as silica gel, amorphous aluminosilicate, or faujasite-type zeolite. A bed of the solid adsorbent is first pretreated with a stream of purified paraffins obtained from a prior purification cycle. The paraffin feedstock is then passed through the bed of solid adsorbent to remove aromatics therefrom until the aromatic content of the effluent reaches a specified level. Desorption of the adsorbed aromatics is carried out at 50.degree.-500.degree. C. using steam, ammonia, isopropyl alcohol, acetone, toluene, or the like. The desorbent must then be removed from the solid absorbent using a gas purge at 200.degree.-500.degree. C., and the bed must consequently be cooled to between 20.degree.-150.degree. C., using either a stream of purified paraffins or a gas, before resuming the adsorption phase.
Commonly owned, co-pending patent application U.S. Ser. No. 07/238,854 filed Aug. 31, 1988 entitled "Process for the Linear Paraffins" is directed to a to a process for purifying a hydrocarbon feedstock which contains linear paraffins, and at least one contaminant selected from the group consisting of aromatic compounds, nitrogen-containing compounds, sulfur-containing compounds, oxygen-containing compounds, color bodies, and mixtures thereof involves a) contacting a liquid feed stream of the hydrocarbon feedstock with an adsorbent comprising a zeolite having an average pore size of from about 6 to about 15 Angstroms under conditions suitable for the adsorption of the at least one contaminant by the zeolite to produce a contaminant-loaded zeolite; and b) desorbing the contaminant-loaded zeolite using a desorbent comprising an alkyl-substituted benzene. In this application, a feed forward control system is used to measure the aromatics and other impurities in the feed and to determine the adsorption cycle times based on a model or historical data which takes into consideration feed aromatics, adsorbent bed capacity, as well as other critical parameters. Feed forward control systems are conventional techniques whereby process control is accomplished by monitoring a variable to predict and control a subsequent related variable. In U.S. Ser. No. 07/238,854, Supercritical Fluid Chromatography (SFC), which involves the use of a supercritical fluid as a mobile phase with a porous silica stationary phase, is used to predict adsorbent bed utilization and to control the switching of adsorbent beds when the adsorbent in the adsorbent beds is predicted to be substantially saturated with aromatics and other impurities.